What is the recommended practice for when these two are at the same position for a measure starting on the left of the page? Are they best stacked (I have a chord symbol at the first beat also and a high note with ledger lines)? which is what I am doing before the first note.
The rehearsal letter actually says "A1,2,4" so takes up more horizontal space than normal.
I did not find anything in Behind Bars or any other book I have that would help.
Thanks.

Measure numbers, rehearsal letters and segno are all "signals", indeed. "segno" is even a formal, forcing-signal.
When using a Rehearsal Letter, the Measure Number disappears, it is because RH is on the higher hierarchy than MN.
I believe that "segno" has the same power over the RH. Why not to omit RH completely? If there is just one "segno", there is absolutely no need to have RH.
If there are multiple "segno"s (as it would be in case in a never-ending-viennese-valse), than "segno after A2" is the right mutual coordination for musicians.

We professional musicians are more flexible than in theory, and clutter on the page is more distracting than anything else.

Yes that would normally be something to consider, thanks. However alongside the Rehearsal letters is the word "Chorus" because there is (the missing) A3 - Chorus elsewhere (in the relative minor key).
Assuming I go with stacking, which should be on top, Segno or Rehearsal? As I have it, Segno is, because then I can put the Rehearsal below with Chorus or whatever. (There is also a tempo indication etc.)